Silver NPs can be synthesized by using a variety of photoinduced or photocatalytic reduction methods. Photochemical synthesis is a clean process which has high spatial resolution, convenience of use, and great versatility. Moreover, photochemical synthesis enables one to fabricate the NPs in various mediums including cells, emulsion, polymer films, surfactant micelles, glasses, etc. Nano-sized silver particles with an average size of 8 nm were prepared by photoinduced reduction using poly (styrene sulfonate)/poly (allylamine hydrochloride) polyelectrolyte capsules as microreactors (44). Moreover, it was demonstrated that photoinduced method could be used for converting silver nanospheres into triangular silver nanocrystals (nanoprisms) with desired edge lengths in 30-120 nm range (45). Particle growth process was controlled using dual-beam illumination of NPs. Citrate and poly (styrene sulfonate) were used as stabilizing agents. In another study, silver NPs were prepared through a very fast reduction of Ag+ by α-aminoalkyl radicals generated from hydrogen abstraction toward an aliphatic amine by the excited triplet state of 2-substituted thioxanthone series (TX-O-CH2-COO− and TX-S-CH2-COO−). Quantum yield of this prior reaction was tuned by substituent effect on thioxanthones, and led to a kinetic control of conversion of silver ion (Ag+) to silver metal (Ag0) (46).
The direct photo-reduction process of AgNO3 in the presence of sodium citrate (NaCit) was carried out with different light sources (UV, white, blue, cyan, green and orange) at room temperature. Sato-Berrú and coworkers (47) have shown that this light-modification process results in a colloid with distinctive optical properties which can be related to the size and shape of the particles. Moreover, Ghosh and colleagues (48) reported a simple and reproducible UV photo-activation method for the preparation of stable silver NPs in aqueous Triton X-100 (TX-100). The TX-100 molecules act as reducing agent and also as NPs stabilizer through template/capping action.
Furthermore, surfactant solution helps to carry out the process of NPs growth in the diffusion controlled way by decreasing the diffusion or mass transfer co-efficient of the system. It also helps to improve the NPs size distributions by increasing the surface tension at the solvent-NPs interface. Huang and coworkers (49) reported the synthesis of silver NPs in an alkaline aqueous solution of AgNO3/carboxymethylated chitosan (CMCTS) using UV light irradiation. CMCTS, a water-soluble and biocompatible chitosan derivative, served simultaneously as a reducing agent for silver cation and a stabilizing agent for the silver NPs. The diameter range of produced silver NPs was 2–8 nm, and they can be dispersed stably in the alkaline CMCTS solution for more than 6 months.